The present invention relates to the preparation of aluminum sheet surfaces to provide the sheet with a surface suitable for use as part of a lithographic printing plate.
It has long been known to be advantageous to form a printing plate by coating a lithographically suitable photosensitive composition onto the surface of an aluminum sheet substrate with subsequent exposure to light through a mask with eventual development. The oleophilic image areas which remain accept and transfer ink during the printing process and the hydrophilic non-image areas accept water or aqueous solutions during printing to repel such greasy inks.
It has long been known that if the surface of the aluminum substrate were grained, either mechanically, for example by use of wire brushes or particulate slurries, or electrochemically by use of electrolytic solutions of acids such as nitric acid, the printing life of a plate may be substantially extended.
Electrolytic graining of aluminum and the electrolytic process has many advantages over mechanical graining. (See, for example, U.S. Pat. Nos. 3,072,546 and 3,073,765). For certain applications, a very fine and even grain is desired. When the aluminum is to be used as a support for lithographic printing plates such characteristics are especially advantageous. A fine and even grain can be obtained in an electrolyte consisting of an aqueous solution of hydrochloric acid, but the current density employed must be kept quite low or pitting of the aluminum surface will take place and, as a result of the low current density, a relatively long period is required to complete the graining. Electrolytic graining of aluminum sheets with hydrochloric or nitric acids is well known in the art as shown by U.S. Pat. Nos. 3,980,539; 3,072,546; 3,073,765; 3,085,950; 3,935,080; 3,963,594 and 4,052,275, among others.
In electrochemical graining, the surface area is greatly expanded thus providing superior lithographic characteristics. However, there is a problem associated with said systems. The surface, although uniform and having a large surface area, is relatively flat. This creates two problems: (1) Poor draw-down of a printing plate in a vacuum frame which results in halation, and (2) poor water carrying capabilities thereby resulting in difficulties in maintaining a wide latitude for the ink-water balance when printing. Both can have disadvantageous consequences in quality printing.
U.S. Pat. No. 4,242,417 teaches a method of graining the surface of an aluminum sheet substrate which comprises first subjecting the substrate to a mechanical graining treatment such as using a wire brush or wet slurry and then etching in a saturated aqueous solution of an aluminum salt of a mineral acid to which optionally up to 10 percent of a mineral acid may be added. Optionally, the graining action of this solution may be aided by electrolysis.
There are some drawbacks to this process. First, due to the slurry graining, the surface is directional in nature which affects printing quality and ink-water balance. Second, the surface is contaminated with microscopic particles used as the abrasive. Third, the process of slurry graining is one of perpetual change. As the brushes are used they become shorter. As the slurry is used it loses abrasivity, thus requiring additions of fresh material. The aluminum surface purity is a function of time due to a continual build-up of Al(OH).sub.3, Al.sub.2 O.sub.3, and particulate aluminum. All this results in a surface fluctuating in quality.
The present invention seeks to retain the beneficial characteristics of electrochemical graining without suffering the detriments of mechanical graining. The surface prepared by the process of the present invention is matted by a chemical etching step and substantially eliminates the directionality in surface etching which is quite evident with mechanical graining. An electrochemical graining step follows the chemical etch to provide a superimposed grain on the etched surface. The result is a surface having an increased surface area with improved capillary wettability which manifests itself in an improved ink-water balance in printing plates produced with this substrate.